Bleaching of sulfate wood pulp



Patented July 4, 1950 UNITED :S OFFICE BLEACHINGi'OF SULFATE WOOD PULP George Day and Benjamin 6. H005, Berlin, N. 11., assignorsto Brown'Gompany, "Berlin, Na Hi, ai corporation of N dDrawinz. Application January 31,1947, Serial No. 725,722

, 1 This inv'entionrelates to the 'bieachinggr'celflul'osic materials, particularlywoodpulp.

'iOne object of the invention late produce 'hlghly "bleached wood pulp's, both sulphate-and 'sulpnite,

"ofex'ceptional physical characteristicsheretofore unobtainable by known-bleaching methods.

further objector the 'inveritionis to produce,

by less complicated, quicker or less eostly treat- 'ment, rueached pulps having physical quail-ties equivalent and even "su erior to those now eb- 'tained.

"oftentimes in as few as three stages; and'normal sulphate pulp values of five-or-siitgstage sequences may very readily be attained in three stages.

In the practice of this invention, chlorine idioxide is utilized as a bleaching reagent under novel and unusual conditions J pi'oductive, in the case of this reagent even in a single stage, of

remarkable jumps in brightness value without loss in other physicals.

While the treatment herein described may be applied as a l'ate'dr final stage in amy pulp bleaching sequence, for realization of'the highest physical valuesinthe novel sulphate palm of this invention, particularly brightness and beater values, the sequence includes hot (140-210" F.) caustic digestion as one stage preparatory to chlorine dioxide treatment and preferably a combination pretreatment, prior to the hot caustic digestion, of pre-bleach and chlorination, wherein the amount of bleaching reagent input (calculated as equivalent chlorine) in the pre-bleach is from -50% of the total equivalent chlorine input of the entire sequence.

The chlorine dioxide treatment in accordance with this invention is carried out under stock density conditions of 4% or less, at a pH of 6 i 1, and at a temperature of at least 140 F.

Typical treatments are as follows:

Example I (sulphate) A softwood kraft pulp was pretreated with 5.5% chlorine at a stock density of 4% for 2 hours at 65 F., and then was washed.

It was then digested with 1 NaOI-I at 14.5%

Maine t-Claim. (o1.:s 10e) stock density for 2 hours at 140 FE,andtl'ien washed.

"As a third and final stage, the pulp was subjected at a stock density of 4% and at a pI-Io'f 5.8 to the action of 1.19% chlorine dioxide based on the bone dry pulp (the chlorine dioxide containing from 3 to 8% chlorine as an impurity) at 18'0'F. for 2 hours, and then washed, anti-chlored with S02 water and again washed.

The bleached pulp had the followingphysical characteristics:

Brightness (General Electric reflectance "'va'lue) Beater value (2 X Mullen (136) +"1-ear This beater value equalled thebeater value of the raw pulp, which was likewise 578.

action of 8% hypochlorite bleach at'8% stock density and '75 F. for one hour and'was then diluted, without intervening washing, to 4% stock density with sufficient chlorine water to give 2.7% chlorine, based on the weight of the bonedry pulp, and maintained for one .hour at F. and then was washed, antichlored with $02 water and again washed. 3o

As a second stage, the pulp was then subjected to a hot caustic treatment with 1'.5% NaOH at I'14'l'5% stock density for two hours at 140'F.an'd

then washed.

. Asa third, and final stage, the pretreated'pulp was subjected at a stock density of 4%to the action of 0.95% chlorine dioxide based on'the bone-dry pulp (the chlorine dioxide containing from 3 to 8% chlorine as an impurity), at pH 5.8 and F. for two hours and then washed, antichlored and again washed.

The bleached pulp had the following physical characteristics:

Brightness (General Electric reflectance value) 91.0

Beater value (2 X Mullen (140) Tear This beater value again equalled the beater value of the raw pulp which was 588.

In this sequence, the initial bleaching agent input, calculated as equivalent chlorine was 35% of the total equivalent chlorine input for the three stages. It has been determined that highest values are attained in the case of pre-bleach treatment of this kind where the pre-bleaching.

value to 535.

i. e., the hypochlorite chlorine equivalent input is from 20 to 50% of the total equivalent chlorine input in the entire sequence. For example, use of 59.8% of the equivalent chlorine input in the hypochlorite pretreatment on the same stock in a sequence otherwise identical with that of Example II, resulted in a reduced brightness value of 88.2 and beatervalue of 557. Further increase to 73% reduced brightness to 85.2 and beater Example III (sulphz'te) A sulphite raw stock 5.5% chlorine at 4% stock density, minutes, then washed.

65 F. for 30 The washed pulp was next treated with 1% of 17.3 Tappi perman. ganate number was subjected to the action of the beater value of the bleached pulp being higher than the beater value of the raw pulp. In general, with sulphite pulps beater values of the bleached pulp will be higher or at the least substantially as high as that of the raw pulp and exceed 420 even when bleached to brightness as high as 88. I

9 Whereas the chlorine ,dioiiide itr' eat'ment, as

given in the above examples, calls for treatment for two hours at 180 F., a range of 140-180" F. is suitable, varying with the time of treatment.

While theexar'nples hereinabove given are for novel pulps of thisinvention of almost spectacularv physical characteristics, it is to be understood that thewtreatment hereof is applicable to any pulp to give increased bleaching efliciency even finflthe production'of pulps of conventional lower NaOI-I at 4% stock density, 65 F. for minutes and then thickened to about 16 to 18% stock density.

As a; final stage, the pretreated pulp was subjected to the action of 0.85% chlorine dioxide based on the bone-dry pulp (the chlorine dioxide containing from 3 to 8% chlorine as an impurity) at a pH of 6.2 and a stock-densityof 4% at 180 1 F. i0r tw0 hours and then washed, antichlored ,with S02 water and again washed. Thebleached sulphite pulp had p the following physical characteristics: 1

I Brightness 88.5

Beater value 2 XMullen (12'7) +Tear (168) 422' This compared with a raw stock beater value ,of408, and a beater value only 387 (117 Mullen; stock bleached by :id'entical first stage and caustic soap treatments 153 Tear) for the same raw followed by a customary present-day third stage treatment with 6.5% soda b1each,' 0.75% NaOH at 10% stock density for almost six hours at F. to 89.0 brightness.

The brightness values above given for kraft pulps of this invention exceed those secured by custornary present-day treatments when the strength of the raw pulp is maintained. Presentday kraft pulp having brightness values even as J highas 86 are obtainable only by five, six or more stage bleaching sequences, whereas the above 'kraft pulps are the result of three-stage treatment. 7

The preferred kraft pulps of this invention have beater values exceeding 550, and, as above' disclosed, ranging as high as 578 or more, while maintaining brightness values above 88 and even as high as above 90. ,.With the sulphite pulp, it will benoted that this process yields a bleached pulp with exceptionally high-beater value and high brightness,

and at a pH-oifiil.

brightness values in the range down to 80.

I We claim:

Process of bleaching sulphate wood pulp which consists (in treating the pulp sequentially, without intervening, washing, with hypochlorite and chlorine, .the equivalent chlorine input on the treatmentwith hypochlorite being from 20-50% of thegtotal equivalent chlorine input 1 of the entire process, then-digesting the pulp with caustic soda at. a, temperatur e between .F. and

210 -F. .and.- finally. treating the pulpinjaque ous .suspensionwith-chlorine dioxide, said final treat.-

mentbeing carried out at a temperature of. at least 140 F., at a stock density not exceeding,4

GEORGE A. BENJAMIN DAY 6. 1100s.

" REFE EN ES CITED p,

hThetfollowing references are of record in th file'of thistpatentz q UNITED STATES"PATENTS OTHER REFERENCES JahnQModern European Pulp Bleaching, Paper Trade Journah Sept; I Tappi Secs., pages 

